docs: def prepare_schemes(self, req):
          """ make sure this client support schemes required by current request
  
          :param pyswagger.io.Request req: current request object
          """
  
          # fix test bug when in python3 scheme, more details in commint msg
          ret = sorted(self.__schemes__ & set(req.schemes), reverse=True)
  
          if len(ret) == 0:
              raise ValueError('No schemes available: {0}'.format(req.schemes))
          return ret